Illumination is used to light objects for seeing, as also for photography, microscopy, scientific purposes, entertainment productions (including theatre, television and movies), projection of images and as backlights of displays. For illumination purposes, systems in the form of point or single dimensional sources of light are used. Such systems have many drawbacks: light intensity is very high at the light source compared to the rest of the room or environment, and thus such light sources are hurtful to the eye. Such sources also cast very sharp shadows of objects, which are not pleasing to the eye, and may not be preferred for applications such as photography and entertainment production. Such sources also cause glare on surfaces such as table tops, television front panels and monitor front panels.
There are systems that act as light sources in the form of a surface. Fluorescent lights for home lighting may be covered by diffuser panels to reduce the glare. These systems are bulky. They are also not transparent. Diffusers and diffuse reflectors, such as umbrella reflectors, are used as light sources for photography and cinematography, but they are only approximations to uniform lighting.
Backlights of flat-panel screens such as LCD screens provide uniform or almost uniform light. One of the prior solutions for backlighting an LCD screen is to have a light guide in the form of a sheet, with some shapes such as dots or prisms printed on it to extract light or by dispersing light diffusing particles in the bulk. The light guide is formed by sandwiching a high refractive index material between two low refractive index materials. The light is guided from one or more ends of the sheet.
These light sources in the form of a surface actually just redirect the light from some other primary light source, in some predetermined emanation pattern. This primary source of light has to be coupled efficiently to the light source in the form of a surface. Light sources used for this application include CCFL tubes, array of LEDs etc.
Two CCFL tubes, one at each end of the sheet may be used. Each CCFL tube is placed inside a reflecting trough having a rectangular cross section. The light from the CCFL tube is coupled to the sheet. The disadvantage of this method is that the light may have to undergo many reflections before it enters the sheet. Each of these reflections cause some wastage of light and leads to inefficiency. Moreover, there is no control on the direction in which the light enters the sheet. Some part of light is reflected back into the CCFL and this leads to further wastage of light. Some part of the light entering the edge of the backlight sheet exits the opposite edge. Part of this light falls upon the opposite CCFL, and is lost, leading to further inefficiency.
Flat screen color displays normally use illumination in the form of white light. The white light falls on the display such as a LCD which uses color filters to depict colors. Color filters reduce efficiency of the display since large amounts of light are absorbed. Another disadvantage is that because of the color filters, the transmittance of the display is very low. To avoid using color filters, colored light needs to be used. This colored light has to be coupled properly to display the correct colors.